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| Report | FZJ-2016-04915 |
2016
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/12453
Report No.: Juel-4396
Abstract: Laser Induced Desorption Spectroscopy (LIDS) is a diagnostic method to measure the hydrogen content in the surface of a material exposed to a hydrogen isotope (H, D, T) plasma. It is developed mainly to monitor hydrogen retention in the walls of magnetic fusion devices that have to limit the amount of their fuel tritium mainly due to safety reasons. The development of fusion increasingly focusses on plasma-wall interactions for which in situ diagnostics like LIDS are required that work during plasma operation and without tile removal. The method has first been developed for thin amorphous hydrocarbon (a-C:H < 500 nm) layers successfully and is studied in the present work on thick (15 μm) layers, carbon fibre composites (CFCs), bulk tungsten (W), W fuzz and mixed C/W materials.In LID a 3 ms Nd:YAG (1064 nm) laser pulse heats a spot of Ø3 mm with 500 MW/m² on W to 1800 K at the surface and thus above 1300 K within ca. 0.2 mm depth. On C materials (graphite, CFC, a-C:H) this temperature guarantees a nearly complete (>95%) desorption already within 1.5 ms pulse duration. The retained hydrogen atoms are desorbed locally, recombine to molecules and migrate promptly to the surface via internal channels like pores and grain boundaries. Whereas, in W the retained hydrogen atoms have to diffuse through the bulk material, which is a relatively slow process also directed into the depth. The desorbed hydrogen fraction can thus be strongly reduced to 18-91% as observed here. This fraction is measured by melting the central part of a previously heated spot ca. 40 μm deep with a Ø2 mm, 3 ms laser pulse, releasing the remaining hydrogen. W samples exposed to different plasmas in TEXTOR, Pilot-PSI, PSI-2, PADOS and PlaQ show that the desorption fraction of LID mainly decreases due to higher sample temperature during plasma exposure. The heat causes deeper hydrogen diffusion and/or stronger hydrogen trapping due to creation of traps with higher binding energy. Such effects can lead to the observed desorption fractions as simulations (TMAP7 code) of heat and H diffusion during the laser pulse show. These experiments are performed in a vacuum chamber outside the tokamak, where the desorbed gases are quantified by a quadrupole mass spectrometer, thus representing the ex situ method LID-QMS.In the tokamak TEXTOR the in situ diagnostic method LIDS is used utilizing the same physics for heating, desorption and surface modifications. Understanding the latter becomes important to mitigate material release into the plasma. Here, the quantification of the desorbed hydrogen is done by passive spectroscopy of the Balmer Hα and Dα light (656 nm) observed coaxially to the laser beam as a double line by a spectrometer and from the side by a camera with gated image intensifier using a narrow-band H&D filter. A simplified data evaluation has been developed which determines the plasma radius of the light intensity maximum of the LIDS light, takes the electron density and temperature at this radius measured by edge plasma diagnostics and looks up the corresponding quotient of ionisation to excitation rate S/XB(ne,Te) in a database (ADAS). A second factor takes into account the dominant plasma processes which yield only one atom from one hydrogen molecule for pure hydrogen release and even less for desorbed hydrocarbons. The combined light-to-particle conversion factor is ca. 30 H atoms/Hα photons which agrees with simulations of the LIDS light (ERO code). While the simulated spatial light distribution is very sensitive to the details of the plasma edge profiles, the total photon amount stays very constant, thus justifying the simplified data evaluation. The experimental FWHM of the light in toroidal/poloidal direction is 30-40 mm and has an e-folding decay length of 15-20 mm in radial direction. Its intensity maximum is typically at ne ≈ 4∙10^18 e−/m³ and kB Te ≈ 60 eV close to the last closed flux surface.A measurement series shows good reproducibility of LIDS with a standard deviation of ±13%, while the estimated uncertainty of a single LIDS measurement is −47% to +43%. LIDS measurements are also in agreement with results from LID-QMS, slow thermal desorption (TDS) or nuclear reaction analysis (NRA). The lower detection limit of LIDS is determined by the Hα background fluctuations in TEXTOR to 8∙10^20 H/m² for ohmic and 5∙10^21 H/m² for neutral beam heated plasmas for a Ø2.6 mm laser spot. The upper measurement limit due to local plasma cooling by the cooler desorbed gas lies at ca. 6∙10^22 H/m² for TEXTOR conditions.
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